Technological advances in the automatic identification industry have provided many industries ways to improve productivity and increase efficiency in many of the processes requiring the collection of data. These industries include inventory control, product flow, quality control, purchasing, and other related business activities. One automated method for collecting data is through the use of devices which read and decode symbol codes. Most people are familiar with the use of scanners for reading bar codes on product labels in retail applications.
A number of different bar code readers and laser scanning systems have been developed to read and interpret bar codes. Optical scanners are available in a variety of configurations including fixed scanning stations and handheld devices. A number of these devices use laser diodes to produce a beam which is scanned across the code to be read and then interprets the reflections returned by the scanning beam. While effective, laser diode scanners are expensive to manufacture and are more susceptible to maintenance problems.
Another type of symbol code reader uses an illumination source in conjunction with an image sensor, typically charge coupled device (CCD). This type of bar code reader is generally referred to as a "CCD scanner" (despite the fact that it does not scan the code in the same fashion a laser diode scanner would). CCD technology is the same technology used in video and digital still cameras. In general a charge coupled device is semiconductor device wherein minority charge is stored in a spatially defined depletion region (potential well) at the surface of a semiconductor and is moved about the surface by transferring this charge to similar adjacent wells. When used as an image sensor, the charges are introduced when the light from a scene is focussed on the surface of the device. The image points are accessed sequentially to produce a television type output signal. While conventional CCD barcode readers are typically less expensive to manufacture they tend to have a very narrow field of view and are unable to adequately illuminate the target symbol to be used in high speed applications. When used as method of capturing data, a light source is used to illuminate the object, and, with the use of optical elements, can recreate the image on the CCD. By capturing the image on the CCD array, the image is converted to an electrical signal and can then be processed and used in a given application. These applications may include reading bar codes, 2-D symbologies, fingerprints, signatures, optical character recognition and many others.
Some applications require reading images at high speed as the image is presented to the reader via a conveyer belt. These applications are prevalent in small package distribution, letter sorting, and industrial and pharmaceutical manufacturing. When scanning at high speed, one of the most difficult problems is not providing high intensity, uniform illumination to cover the target area. The two most commonly used devices are flash strobes and light emitting diodes (LEDs). The flash strobe is capable of providing enough light to be used in high speed applications where the exposure time is very short. However, because flash strobes are relatively expensive and have short life span, they are only used in expensive systems. In addition, flash strobes require high voltage to operate and, as a result, need warning labels to alert personnel of the danger. The LED is normally used in low speed applications since it does not provide sufficient light, nor is it uniform within the target area, henceforth referred to as the field of view (FOV) or field of illumination.
There are many devices and methods already well known in the prior art to provide illumination for automatic identification or bar code scanning. U.S. Pat. No. 4,315,245 teaches the use of two linear arrays of LEDs to illuminate a target area, with a control circuit used to vary the amount of illumination. This method, however, does not produce a uniform 2-D illumination, nor is the light homogenous from the light-emitting diodes. U.S. Pat. No. 5,192,856 teaches a method for producing a 1-D or 2-D area of illumination for scanning applications. However, this method lacks sufficient intensity for high speed applications. U.S. Pat. No. 5,532,467 teaches a method of using a linear array of LEDs providing a fan of light. However, this is only suitable for 1-D bar code scanning as it does not illuminate a sufficient 2-D area to allow 2-D symbologies to be read.
There is a need for an LED-based scanning device which provides adequate illumination for both 1-D bar codes and 2-D symbologies for both hand-held and fix-mount applications. There is a further need for method and device which is suitable for applications involving high speed scanning in which high intensity illumination must be provided during very short exposure times. There is a yet a further need for device which provides highly uniform illumination over the target area. Finally, there is a need a device which provides linear polarized light for applications in which highly specular targets must be illuminated and imaged with high contrast.